Spreadsheet programs, such as LOTUS, QUATRO and EXCEL, are powerful tools commonly used for collecting, analyzing, and managing data in an efficient and expedient manner. Mathematical applications such as record-keeping, optimization analysis, calculations, charting and graphical analysis are examples of typical uses for spreadsheets.
Spreadsheet programs use spreadsheets for displaying and managing data. Spreadsheets typically include cells arranged in rows and columns. For example, the columns of a spreadsheet may be organized by letter, such as columns A, B, C, etc., and the rows may be organized by number, such as rows 1, 2, 3, etc. Each cell is identified by a combination of column letter and row number. Numerical and textual data can be entered into the cells of the spreadsheet according to the organization imposed by the end user. Also, the cells can be formatted to present the numbers and text in a desired appearance.
In addition to entering data in individual cells, conventional spreadsheet programs also permit the end user to define mathematical functions within individual cells. It is common for the mathematical functions entered in one cell to use data contained in other cells. The function itself is not displayed in the cell. Instead, the cell displays the results of the function.
Typically, several well-used functions are defined by the spreadsheet program. For example, summation of cells in a column may be represented by “=SUM(A3:A5)”. The end user may type this function in cell A6, which results in cell A6 displaying the sum of the data contained in cells A3-A5. Instead of using a predefined function, the end user may type “=A3+A4+A5” in the cell A6 to achieve the same result.
Conventionally, a spreadsheet program creates a dependency tree for the cells and resolves mathematical functions (i.e., equations) for each cell in a linear sequence. For example, assuming cell A6 is the sum of cells A3-A5, if the value in any of the cells A3-A5 changes, the spreadsheet program recalculates the sum and the value in cell A6 changes. The recalculation of a cell is only performed when one of its dependent cells, such as cells A3-A5, changes. When there are no changes in the spreadsheet, no recalculations are performed. However, this conventional recalculation technique precludes the recalculation of the spreadsheet at regular intervals. Also, in environments where time-dependent circular references are desirable, such as sensor-driven control systems where actions are taken after aggregating sensor data and time-dependent variables, the use of a spreadsheet for managing and controlling the system is generally not feasible due to the lack of support for circular, time-dependent, references.
In addition, a spreadsheet provides a tool for analyzing and managing data. However, the spreadsheet provides limited information about data sources supplying data to the spreadsheet unless the information is input into the spreadsheet. When a large amount of data is being managed, it is costly and time consuming to input information about the data sources in the spreadsheet.